Voltage detector circuit with a programmable threshold point

ABSTRACT

A voltage detector including a voltage following circuit connected to a power supply and operable to follow a voltage value of the power supply, a selectable threshold point circuit connected to the voltage following circuit and operable to select one of a plurality of values for a threshold point of the power supply, and a switch circuit coupled to the selectable threshold point circuit and the voltage following circuit, the switch circuit cooperating with the selectable threshold point circuit to generate an output indicating whether the value of the power supply has increased above or decreased below the selected value for the threshold point in response to the followed value of the power supply.

CROSS-REFERENCE TO RELATED APPLICATION AND CLAIM OF PRIORITY

The present application is a continuation of U.S. patent applicationSer. No. 09/780,208, filed on Feb. 9, 2001 now abandoned, entitled“Voltage Detector Circuit With A Programmable Threshold Point,” which isassigned to the present assignee and hereby incorporated by reference inits entirety.

BACKGROUND

1. Field of the Invention

The present invention relates generally to voltage detectors and, moreparticularly, to a voltage detector circuit with a programmablethreshold point.

2. Related Art

Battery-powered portable electronic devices, such as laptop computersand cell phones, require circuits to detect power-on and low powerconditions of their battery power supplies. Without such circuits, theportable electronic devices may operate improperly or fail.

Circuits that detect power-on conditions are commercially available inthe form of power supply monitoring chips. These chips typically havepower supply voltage following circuits that track the power supplyvoltage and output a signal during turn-on when the power supply voltagesurpasses a pre-determined threshold point on the rising-edge of thepower supply voltage. The threshold point signifies a voltage sufficientfor device operation and serves to provide safe startup of the devicesby indicating when the power supplies have stabilized at acceptablevoltage levels.

Circuits that detect low power conditions are also commerciallyavailable in the form of microprocessor reset chips. These chips trackthe power supply voltage and output a signal when the power supplyvoltage drops below a threshold point. In this case, the threshold pointsignifies that the power supply voltage has fallen to a level which isinsufficient for device operation or that the power supply voltage isdecreasing toward a critically low level.

One problem with previously developed technologies is that there are nochips having circuits that detect both the power-on and low powerconditions. While it is possible to incorporate both a power monitoringcircuit and a microprocessor reset circuit on a single chip, doing sowould require a substantial amount of surface area for the chip.Additionally, connecting the two circuits unduly increases thecomplexity of the chip.

Another problem with previously developed technologies is that theindividual power monitoring circuit chips and microprocessor resetcircuit chips themselves are too large to fit inside small portableelectronic devices, which are becoming smaller and smaller. One reasonfor this is that both types of chips require a circuit that generates areference voltage from which a threshold point value is derived. Thisreference voltage generation circuit is relatively large and thusincreases the size of the individual chips.

Another problem with previously developed technologies is thatcommercial reset chips only provide a small number of programmablethreshold point values (e.g., 2 or 3). Chips offering a large number ofthreshold point values have preset levels for the threshold point valuesand are only available as discrete components.

It is also important that the amount of power or current consumed bycircuits or chips in battery-powered portable electronic devices is low.This ensures that the battery-powered portable electronic devices canoperate for commercially acceptable periods of time.

Accordingly, what is needed is a circuit that can be implemented on asingle chip, that is capable of detecting both power-on and low powerconditions, that can be programmed to detect a large number of thresholdpoints values, and that consumes a low amount of current.

SUMMARY OF THE INVENTION

The present invention provides a detector that can be implemented on asingle chip, that is capable of detecting both power-on and low powerconditions, that can be programmed to detect a large number of thresholdpoint values, and that consumes a low amount of current.

In one embodiment of the present invention, a voltage detector isdisclosed. The voltage detector includes a voltage following circuitconnected to a power supply and operable to follow a voltage value ofthe power supply, a selectable threshold point circuit connected to thevoltage following circuit and operable to select one of a plurality ofvalues for a threshold point of the power supply, and a switch circuitcoupled to the selectable threshold point circuit and the voltagefollowing circuit, the switch circuit cooperating with the selectablethreshold point circuit to generate an output indicating whether thevalue of the power supply has increased above or decreased below theselected value for the threshold point in response to the followed valueof the power supply.

In another embodiment of the present invention, a method for detecting avoltage level performed in a circuit is disclosed. The method includesselecting one of a plurality of values for a threshold point for a powersupply, tracking a voltage value of the power supply, and generating anoutput that indicates whether the voltage value of the power supply hasincreased above or decreased below the selected value for the thresholdpoint in response to the tracked value of the power supply.

In another embodiment of the present invention, a system including amemory, a microprocessor, and a voltage detector coupled to the memoryand the microprocessor is disclosed. The voltage detector includes avoltage following circuit connected to a power supply and operable tofollow a voltage value of the power supply, a selectable threshold pointcircuit connected to the voltage following circuit and operable toselect one of a plurality of values for a threshold point of the powersupply, and a switch circuit coupled to the selectable threshold pointcircuit and the voltage following circuit, the switch circuitcooperating with the selectable threshold point circuit to generate anoutput indicating whether the value of the power supply has increasedabove or decreased below the selected value for the threshold point inresponse to the followed value of the power supply.

Other aspects and advantages of the present invention will becomeapparent from the following descriptions and accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention and forfurther features and advantages, reference is now made to the followingdescription taken in conjunction with the accompanying drawings, inwhich:

FIG. 1 is a block diagram of a system including a voltage detector, amemory, and a microprocessor, according to an embodiment of the presentinvention.

FIG. 2 is a schematic diagram of a circuit implementation for thevoltage detector of FIG. 1, according to an embodiment of the presentinvention.

FIGS. 3A-3C are diagrams illustrating the response of the voltagedetector of FIG. 2 to a varying supply voltage input.

FIG. 4 is a schematic diagram of a current source generator block,according to an embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The preferred embodiments of the present invention and their advantagesare best understood by referring to FIGS. 1 through 4 of the drawings.Like numerals are used for like and corresponding parts of the variousdrawings.

FIG. 1 is a block diagram of a system 10 including a voltage detector100, a memory 106, and a microprocessor 118, according to an embodimentof the present invention. Voltage detector 100 can be implemented on asingle chip, is capable of detecting both power-on and low powerconditions, can be programmed to detect a large number of thresholdpoint values, and consumes a low amount of current.

Voltage detector 100 is connected to a power supply (or supply voltage)Vdd and a ground GND. Voltage detector 100 is also connected to memory106, such as an electrically programmable read-only memory (EEPROM), andmicroprocessor 118. Memory 106 stores a plurality of bits whichcorrespond to and define a number of values for a threshold voltage thatcan be programmed in detector 100. As used herein, the term “thresholdvoltage” refers to a value of supply voltage Vdd which is less than itsmaximum value and at which supply voltage Vdd may be considered to be ateither a low power or power-off condition. For each threshold voltagevalue, a respective set of data bits may be provided in memory 106. Theplurality of bits are conveyed from memory 106 to a selectable thresholdpoint circuit block 108 within voltage detector 100 via a plurality ofcontrol signals N. In one embodiment, a separate control signal may beprovided to selectable threshold point circuit 108 for each bit of a bitset. The number of bits (or control signals N) determines the resolutionof the programmable threshold voltage. The use of more data bits in eachbit set will allow more values to be defined, and thus provide greaterresolution.

Selectable threshold point block 108 is connected to a switch circuit ata node CC, a voltage following circuit board 107 at node AA, and to amemory 106. Selectable threshold point circuit block 108 generallyfunctions to select one of a number of values for the threshold voltageat which supply voltage Vdd is deemed to be at a low power or power-offcondition. When the supply voltage Vdd has a value lower than theselected threshold voltage value, then selectable threshold pointcircuit block 108 may pull the voltage at node CC up to the level ofVdd.

Voltage following circuit block 107 is connected to supply voltage Vdd,selectable threshold point circuit block 108 at a node AA, and a currentsource generator at a node BB. Voltage following circuit block 107follows or tracks supply voltage Vdd. Due to the operation of voltagefollowing circuit board 107, the voltage at node BB follows supplyvoltage Vdd.

Current source generator block 110 is connected to node BB and supplyvoltage Vdd. Current source generator block 110 generally functions togenerate a current which is provided to node BB.

A switch circuit block 109 is connected to voltage following circuitblock 107 and current source generator block 110 at node BB, and toselectable threshold voltage circuit block 108 at a node CC. Switchcircuit block 109 generally functions as a switch. When supply voltageVdd has a value greater then the selected threshold voltage, switchcircuit block 109 pulls the voltage of node CC to ground.

A voltage level detection circuit block 116 is connected to node CC andexternal microprocessor 118. Voltage level detection circuit block 116generally functions to output a signal which indicates to microprocessor118 whether the supply voltage Vdd is at a low power condition or apower-on condition. This is further described herein.

In operation, one of the bit sets stored in memory 106 is conveyed toselectable threshold point circuit block 108 via control signals N. Thebit set essentially programs the selectable threshold point circuit 108,thus selecting the threshold voltage value associated with that data bitset. From another perspective, the control signals define the magnitudeof a pull-up current that flows from node CC through selectablethreshold point circuit block 108. Voltage following circuit block 107tracks the power supply voltage Vdd and outputs a tracked voltage whichappears at node BB, the input to switch circuit block 109. The trackedvoltage controls the magnitude of a pull-down current that flows fromnode CC through switch circuit block 109. Node CC functions as adetection node for the threshold point.

When the magnitude of the voltage supply Vdd is lower than the selectedthreshold point, the pull-down current flowing through switch circuitblock 109 is less than the pull-up current flowing through selectablethreshold point circuit block 108. This pulls the voltage level of nodeCC to the voltage level of supply voltage Vdd through selectablethreshold point circuitry block 108. The voltage level detection circuitblock 116 detects this voltage value at node CC and, in response,outputs a signal which tracks the supply voltage Vdd. This signifiesthat the value of supply voltage Vdd is below the threshold point.

When the magnitude of the voltage supply Vdd is greater than theselected threshold point, the pull-down current flowing through switchcircuit block 109 is greater than the pull-up current flowing throughselectable threshold point circuit board 108. This pulls the voltagelevel of node CC to ground through switch circuit block 109. The voltagelevel detection circuit block 116 then detects this low voltage at nodeCC and, in response, outputs a driven and clearly defined low output tomicroprocessor 118. This signifies that the value of the voltage supplyVdd is above the threshold point.

FIG. 2 is a schematic diagram of a circuit implementation for voltagedetector 100 of FIG. 1, according to an embodiment of the presentinvention. In particular, FIG. 2 depicts a number of circuits whichcorrespond to voltage following circuit block 107, selectable thresholdpoint circuit block 108, switch circuit block 109, current sourcegenerator block 110, and voltage level detection circuit block 116.

Voltage following circuit block 107 includes a weak NMOS transistor 230configured as a source follower. Voltage following circuit block 107functions to track supply voltage Vdd. The gate terminal of transistor230 is connected to supply voltage Vdd. Supply voltage Vdd is providedby power source 204, which can be a battery. The source terminal oftransistor 230 is connected to node BB. The drain terminal of transistor230 is connected to selectable threshold voltage point block 108 at nodeAA.

Selectable threshold point circuit block 108 includes a number of PMOScurrent mirror transistors 232, 234, 236, 238, and 240, which functionas current mirrors, coupled to a number of NMOS switch transistors 242,244, 246, and 248, which function as switches. Selectable thresholdpoint circuit block 108 functions to precisely set the value of thethreshold voltage. A PMOS transistor 228 provides a reference for thecurrent mirrors. The source and body terminals of current-referencetransistor 228 are connected to supply voltage Vdd. The gate and drainterminals of current-reference transistor 228 are connected tocurrent-mirror-reference node AA. The gate terminals of current mirrortransistors 232, 234, 236, 238, and 240 are connected to current mirrorreference node AA. The source and body terminals of current mirrortransistors 232, 234, 236, 238, and 240 are connected to supply voltageVdd. The drain terminal of current mirror transistor 232 is connected tonode CC. The drain terminals of current mirror transistors 234, 236,238, and 240 are connected to the drain terminals of switch transistors242, 244, 246, and 248, respectively. The source terminals of switchtransistors 242, 244, 246, and 248 are connected to node CC. The gateterminals of switch transistors 242, 244, 246, and 248 are connected tomemory 106 to receive control signals 208, 210, 212 and 214,respectively, which are generated from bit sets in memory 106.

The selected value of the threshold voltage is determined by the amountof current which is allowed to flow from selectable threshold voltagecircuit block 108 to node CC. If more current is allowed to flow, thenthe value selected for the threshold voltage will be higher. Conversely,if less current is allowed to flow, then the value selected for thethreshold voltage will be lower.

In operation, when any of the control signals 208, 210, 212 and 214 havea logic high value, the respective switch transistor 242, 244, 246,and/or 248 is turned on. This allows current to flow through therespective current mirror transistor 234, 236, 238, and/or 240 to nodeCC. When any of control signals 208, 210, 212 and/or 214 have a logiclow value, the respective switch transistor 242, 244, 246, and/or 248 isturned off. This prevents current from flowing through the respectivecurrent mirror transistor 234, 236, 238, and/or 240 to node CC. Notethat the current always flows through current-mirror transistor 232 tonode CC since it is not controlled by a switch transistor.

The amount of current conducted by each individual current mirrortransistor 232, 234, 236, 238, and 240 is determined by its width tolength (W/L) ratio. In one embodiment, current mirror transistors 232,234, 236, 238, and 240 each have a different W/L ratio. Thus, the totalamount of current that flows to node CC from selectable threshold pointcircuit block 108 can be precisely set and controlled depending on whichswitch transistors 242, 244, 246, and 248 have been turned on byrespective control signals 208, 210, 212 and 214 (according to aparticular data bit set).

In one embodiment, switch transistors 242, 244, 246, and 248 have W/Lratios of 5μ/2μ. Current-mirror reference transistor 228 andcurrent-mirror transistor 232 have W/L ratios of 8μ/4μ, current-mirrortransistor 234 has a W/L ratio of 3μ/4μ, current-mirror transistor 236has a W/L ratio of 6μ/4μ, current-mirror transistor 238 has a W/L ratioof 8μ/4μ, and current-mirror transistor 240 has a W/L ratio of 10μ/4μ.

Switch circuit block 109 includes a weak NMOS transistor 112. Switchcircuit block 109 allows node CC to be pulled up to the voltage level ofsupply voltage Vdd or down to ground GND depending on the amount ofcurrent being provided to node CC by selectable threshold point circuitblock 108 and the value of the tracked supply voltage appearing at nodeBB. The gate terminal of transistor 112 is connected to node BB. Thedrain terminal of transistor 112 is connected to node CC. The sourceterminal of transistor 112 is connected to ground GND. When the value ofsupply voltage Vdd is above the selected value for the thresholdvoltage, transistor 112 is turned on, and the voltage at node CC ispulled low.

Current source generator block 110 includes an NMOS transistor 250(which can be a depletion type transistor), NMOS transistors 252 and254, and a capacitor 256. Transistor 252 is configured as adiode-connected current reference transistor and transistor 254 isconfigured as a current mirror. The drain terminal of transistor 250 isconnected to supply voltage Vdd. The gate and source terminalstransistor 250, the gate and drain terminals of transistor 252, and thegate terminal of transistor 254 are connected together at a node DD. Thesource terminals of transistors 252 and 254 as well as one terminal ofcapacitor 256 are connected to ground GND. The other terminal ofcapacitor 256 and the drain terminal of transistor 254 are connected tonode BB. The gate-source connected transistor 250 sets the current levelpassed to current reference transistor 252. This current level isrelatively constant and independent of voltage source 204.

In operation, transistor 250 provides current to current referencetransistor 252. This current is proportionately mirrored in currentmirror transistor 254. The mirrored current, acting against the pull-upcurrent provided by PMOS transistor 228 (in the selectable thresholdvoltage circuit block 108) and weak NMOS transistor 230 (in thevoltage-following circuitry block 107), sets the voltage at the node BB,that is, the gate voltage of weak transistor 112 (in switch block 109).The voltage at node BB increases with an increasing power supply voltageVdd. Capacitor 256 stabilizes the voltage at node BB.

In one embodiment, NMOS transistors 252 and 254 both have W/L ratios of4μ/4μ and capacitor 256 has a capacitance of 500fF.

Voltage level detection circuit block 116 includes PMOS transistors 260,262, and 264, NMOS transistors 266, 268, and 270, and an inverter 272.The source and body terminals of transistors 260 and 262 are connectedto supply voltage Vdd. The gate terminal of transistor 262 is connectedto node CC. The drain terminal of transistor 262 is connected to thesource terminal of transistor 264 and to the drain terminal oftransistor 260. The body terminal of transistor 264 is connected tosupply voltage Vdd. The gate terminal of transistor 264 is connected tonode CC. The drain terminal of transistor 264 is connected to the drainterminal of transistor 266 and to the input terminal of inverter 272.The gate terminal of transistor 266 is connected to node CC. The sourceterminal of transistor 266 is connected to the drain terminals oftransistors 268 and 270. The gate terminal of transistor 268 isconnected to node CC. The source terminals of transistors 268 and 270are connected to ground GND. The gate terminals of transistor 270 andtransistor 260 are connected to the output terminal of inverter 272. Theinverter 272 outputs an output signal 274. Transistors 262, 264, 266,and 268 are connected as an inverter. This inverter, in combination withinverter 272, forms a hysteresis circuit. Transistors 260 and 270provide a feedback loop.

In one embodiment, transistor 268 has a W/L ratio of 10μ/2μ, transistor266 has a W/L ratio of 7μ/1μ, and NMOS transistor 270 has a W/L ratio of12μ/1μ. Transistor 262 has a W/L ratio of 20μ/2μ, transistor 264 has aW/L ratio of 15μ/1μ, and transistor 260 has a W/L ratio of 24μ/1μ.

In operation, voltage level detection circuit block 116 monitors thevoltage at node CC, which indicates when supply voltage Vdd is less thanor greater than the programmed threshold voltage. The threshold pointfor supply voltage Vdd is reached when the pull-up current from theselectable threshold point circuit block 108 is equals the pull-downcurrent in transistor 112. If the magnitude of the supply voltage Vdd islower than the threshold point, the current flowing through selectablethreshold point circuit block 108 pulls the voltage at node CC up to thelevel of Vdd. This indicates a low power condition. In contrast, if themagnitude of the supply voltage Vdd is greater than the threshold point,the current flowing through transistor 112 pulls the voltage at node CCto ground GND. This indicates a sufficient/adequate power condition. Thevoltage at node CC is put through the hysteresis circuit (consisting oftwo inverters) and a feedback loop to sharpen the output signal and toprevent it from responding to small signal perturbations on the node CCat the threshold point.

FIGS. 3A-3C are diagrams illustrating the response of the voltagedetector of FIG. 2 to a varying supply voltage input. FIG. 3A shows themagnitude of the supply voltage Vdd and the magnitude of the voltage atnode BB on the y-axis versus time on the x-axis. FIG. 3B shows themagnitude of the output signal 274 appearing at the output terminal ofvoltage detector 100 on the y-axis versus time on the x-axis when anexemplary threshold point B has been programmed. FIG. 3C shows themagnitude of the output signal 274 appearing at the output terminal ofvoltage detector 100 on the y-axis versus time on the x-axis when anexemplary threshold point A has been programmed. Although FIGS. 3B and3C only shows the response for two programmed threshold points,threshold point A and threshold point B, skilled artisans will recognizethat a large number of programmable threshold voltages can be preciselydefined using selectable threshold point circuit block 108.

Initially, as the level of supply voltage Vdd increases from 0.0 volts,both the voltage at node BB and the voltage at the output terminal ofvoltage detector (output signal 274) increase proportionally. When Vddexceeds the programmed threshold point (approximately 2.5 volts forthreshold point B and 4.3 volts for threshold point A), the voltage atnode BB increases proportionally, but the voltage at the output terminalof voltage detector 100 (output signal 274) falls to approximately 0.0volts. This signifies a power-on condition.

As long as the magnitude of the supply voltage Vdd is above thethreshold point, the voltage at the output terminal of voltage detector100 remains at approximately 0.0 volts. But, as the magnitude of supplyvoltage Vdd decreases from 5.0 volts, the voltage at node BB decreasesproportionally. When the magnitude of the supply voltage Vdd decreasesbelow the programmed threshold point (approximately 2.5 volts forthreshold point B and 4.3 volts for threshold point A), the voltage atthe output terminal of voltage detector 100 rises from 0.0 volts to theprogrammed threshold point. This indicates a low voltage condition. Thenas the magnitude of the supply voltage Vdd continues to decrease, boththe voltage at node BB and the voltage at the output terminal of voltagedetector 100 decrease proportionally.

FIG. 4 is a schematic diagram of a current source generator block 210,according to an embodiment of the present invention. Current sourcegenerator block 210 can be used as an alternative to current sourcegenerator block 110 of FIG. 2.

Current source generator block 210, like the current source generatorblock 110, includes NMOS transistor 250, NMOS transistors 252 and 254,node DD, and capacitor 256. Current source generator block 210 furtherincludes NMOS transistors 300, 302, 304, and 306. NMOS transistors 300and 302 are connected in series, as are NMOS transistors 304 and 306.The source terminals of NMOS transistors 302 and 306 are connected toground GND. The gate terminals of NMOS transistors 302 and 306 areconnected to node DD. The drain terminals of NMOS transistors 300 and304 are connected to node BB. The gate terminals of NMOS transistors 300and 304 are connected to receive control signals 308 and 310,respectively. Control signals 308 and 310 may be generated from datastored in memory 106.

Transistor 252 functions as a current reference and each of transistors254, 302, and 306 function as current mirrors. Transistors 300 and 304function as switches for current mirror transistors 302 and 306,respectively.

In operation, when any of control signals 308 or 310 have a logic highvalue, the respective switch transistor 300 and/or 304 is turned on.This allows current to flow through the respective current mirrortransistor 302 and/or 306 to node BB. When any of control signals 308 or310 have a logic low value, the respective switch transistor 300 and/or304 is turned off. This prevents current from flowing through therespective current mirror 302 and/or 306 to node BB.

While particular embodiments of the present invention and theiradvantages have been shown and described, it should be understood thatvarious changes, substitutions, and alterations can be made thereinwithout departing from the spirit and scope of the invention as definedby the appended claims.

1. A voltage detector comprising: a voltage following circuit connectedto a power supply and operable to follow a voltage value of the powersupply; a selectable threshold point circuit connected to the voltagefollowing circuit and operable to provide one of a plurality of valuesfor a threshold point of the power supply without using a referencevoltage; and a switch circuit coupled to the selectable threshold pointcircuit and the voltage following circuit, the switch circuitcooperating with the selectable threshold point circuit to generate anoutput indicating whether the value of the power supply has increasedabove or decreased below the provided value for the threshold point inresponse to the followed value of the power supply.
 2. The voltagedetector of claim 1 wherein the selectable threshold point circuit isoperable to receive a plurality of control signals.
 3. The voltagedetector of claim 1 wherein the selectable threshold point circuit isoperable to output a programmable amount of current.
 4. The voltagedetector of claim 1 wherein the selectable threshold point circuitcomprises a plurality of current mirror transistors.
 5. The voltagedetector of claim 4 wherein at least one of the current mirrortransistors is coupled to a respective switch transistor.
 6. The voltagedetector of claim 5 wherein the switch transistor is operable to receivea control signal.
 7. The voltage detector of claim 1 wherein theselectable threshold point circuit comprises a plurality of currentmirror transistors, at least two of the current mirror transistorshaving a different width-to-length ratio.
 8. The voltage detector ofclaim 1 wherein the switch circuit comprises a transistor.
 9. Thevoltage detector of claim 8 wherein a gate of the transistor receivesthe followed value of the power supply.
 10. The voltage detector ofclaim 1 wherein the switch circuit and the selectable threshold pointcircuit are connected at a detection node, the switch circuit operableto pull a voltage at the detection node to ground when the value of thepower supply is above the provided value for the threshold point. 11.The voltage detector of claim 1 wherein the switch circuit and theselectable threshold point circuit are connected at a detection node,the selectable threshold point circuit operable to pull a voltage at thedetection node up to the value of the power supply when the power supplyis below the provided value for the threshold point.
 12. The voltagedetector of claim 1 further comprising a current source generator blockcoupled to the voltage-following circuit and the switch circuit.
 13. Thevoltage detector of claim 12 wherein the current source generator blockcomprises: a reference transistor; and a current mirror transistorcoupled to the reference transistor and the switch circuit.
 14. Thevoltage detector of claim 12 wherein the current source generator blockcomprises: a reference transistor; and a plurality of current mirrortransistors coupled to the reference transistor and the switch circuit.15. The voltage detector of claim 1 further comprising a voltage leveldetection circuit coupled to the selectable threshold point circuit andthe switch circuit, the voltage level detection circuit operable tooutput a signal indicating whether the value of the power supply isabove or below the provided value for the threshold point.
 16. A methodfor detecting a voltage level performed in a circuit, the methodcomprising: providing one of a plurality of values for a threshold pointfor a power supply without using a reference voltage; tracking a voltagevalue of the power supply; and generating an output that indicateswhether the voltage value of the power supply has increased above ordecreased below the provided value for the threshold point in responseto the tracked value of the power supply.
 17. The method of claim 16wherein providing comprises transmitting at least one control signal tothe circuit.
 18. The method of claim 16 wherein providing comprisesturning on at least one switch transistor.
 19. The method of claim 16wherein generating comprises pulling a voltage level at a detecting nodeto ground when the value of the power supply exceeds the providedthreshold point.
 20. The method of claim 16 wherein generating comprisespulling a voltage level at a detecting node to the value of the powersupply when the value of the power supply is below the providedthreshold point.
 21. A system comprising: a memory; a microprocessor;and a voltage detector coupled to the memory and the microprocessor, thevoltage detector comprising: a voltage following circuit connected to apower supply and operable to follow a voltage value of the power supply;a selectable threshold point circuit connected to the voltage followingcircuit and operable to provide one of a plurality of values for athreshold point of the power supply without using a reference voltage;and a switch circuit coupled to the selectable threshold point circuitand the voltage following circuit, the switch circuit cooperating withthe selectable threshold point circuit to generate an output indicatingwhether the value of the power supply has increased above or decreasedbelow the provided value for the threshold point in response to thefollowed value of the power supply.
 22. The system of claim 21 whereinthe selectable threshold point circuit is operable to receive aplurality of control signals.
 23. The system of claim 21 wherein theselectable threshold point circuit is operable to output a programmableamount of current.
 24. The system of claim 21 wherein the selectablethreshold point circuit comprises a plurality of current mirrortransistors.
 25. The system of claim 24 wherein at least one of thecurrent mirror transistors is coupled to a respective switch transistor.26. The system of claim 25 wherein the switch transistor is operable toreceive a control signal.
 27. The system of claim 21 wherein theselectable threshold point circuit comprises a plurality of currentmirror transistors, at least two of the current mirror transistorshaving a different width-to-length ratio.
 28. The system of claim 21wherein the switch circuit comprises a transistor.
 29. The system ofclaim 28 wherein a gate of the transistor receives the followed value ofthe power supply.
 30. The system of claim 21 wherein the switch circuitand the selectable threshold point circuit are controlled at a detectionnode, the switch circuit operable to pull a voltage at the detectionnode to ground when the value of the power supply is above the providedvalue for the threshold point.
 31. The system of claim 21 wherein theswitch circuit and the selectable threshold point circuit are connectedat a detection node, the selectable threshold point circuit operable topull a voltage at the detection node up to the value of the power supplywhen the power supply is below the provided value for the thresholdpoint.
 32. The system of claim 21 further comprising a current sourcegenerator block coupled to the voltage-following circuit and the switchcircuit.
 33. The system of claim 32 wherein the current source generatorblock comprises: a reference transistor; and a current mirror transistorcoupled to the reference transistor and the switch circuit.
 34. Thesystem of claim 32 wherein the current source generator block comprises:a reference transistor; and a plurality of current mirror transistorscoupled to the reference transistor and the switch circuit.
 35. Thesystem of claim 21 further comprising a voltage level detection circuitcoupled to the selectable threshold point circuit and the switchcircuit, the voltage level detection circuit operable to output a signalindicating whether the value of the power supply is above or below theprovided value for the threshold point.